(1) Field of the Invention
This invention relates to a method for diffusing dopant atoms such as aluminum, gallium, phosphorus or the like thermally into the interior of a semiconductor wafer of silicon.
(2) Description of the Prior Art
The thermal diffusion technique is currently used in the fabrication of integrated circuits, large-scale integrated circuits and the like so as to form layers doped with dopant atoms. The thermal diffusion technique permits to form a junction of a doped layer in a silicon wafer. It also enables to form such a junction at a greater depth of the wafer.
As a method for causing dopant atoms to undergo thermal diffusion into the interior of a silicon wafer, it has heretofore been carried out to heat the silicon wafer with the dopant atoms in a resistive furnace. It is however necessary, according to the above conventional method, to carry out the heating of the silicon wafer at a temperature of about 1100.degree. C. and for a time period as long as about 10 hours in order to achieve a desired level of diffusion, although the diffusing temperature and time may vary to certain extents depending on the sort of dopant atoms to be diffused. Moreover, it is also required to conduct the temperature-raising step of the silicon wafer from room temperature to a diffusing temperature where the dopant atoms are caused to undergo diffusion, and the temperature-lowering step of the same silicon wafer from the diffusing temperature to room temperature respectively before and after the diffusion treatment slowly, each, over a time period of 1 hour or longer, thereby making the overall diffusion treatment very time-consuming.
The above-described thermal diffusion method making use of a resistance furnace is superior in that a number of wafers may be subjected to a diffusion treatment at once and, because wafers are heated for a long period of time making use of the thermal conduction by means of the surrounding gas, temperature variations may be kept relatively small at various points of each wafer. The above conventional method is however accompanied by such drawbacks that the wafers often develop crystal defects because they are kept for a long time period at a high temperature and wafers may develop other physical defects such as warping if they are charged or discharged at a higher speed into or from a furnace with a view toward making the treatment time shorter.
With the foregoing in view, it may be contemplated to effect the diffusion at a higher temperature so as to shorten the treatment time. However, the diffusing temperature is said to be 1200.degree. C. or so at most, because the heat treatment of silicon wafers is generally conducted while holding them in a container formed of a high-purity silicon glass tube and any temperatures higher than the melting point of silicon must thus be avoided. Use of phosphorus as a dopant still requires a considerably long time period, i.e., 4 hours for its diffusion even if silicon wafers are heated at a treatment temperature of 1200.degree. C. Furthermore, the problem of developing physical defects such as warping remain unsolved even if the diffusing treatment is carried out at such a high temperature.
When increasing the depth of diffusion, for example, to 10 .mu.m or so from the surface of a wafer by the above-described thermal diffusion method making use of a resistance furnace, it is necessary to make the diffusing temperature still higher and the treatment time still longer. The above-mentioned drawbacks become serious problems if the diffusing temperature and time are increased. Therefore, the conventional diffusion method, which uses a resistance furnace, cannot be considered as a meritorious method in view of its productivity and production yield.